This invention relates to screw presses and particularly to high pressure expressing presses capable of continuous operation, and to improvements in both operating efficiency and reduced maintenance time. Screw presses, which are used to express oils or fluids from fibrous materials such as sugar cane, meals, nuts, bagasse, rubber, and the like, include a main body or cage with drainage openings through the cage side walls, and a feed worm rotationally driven within the cage to express the fibrous contents therein. Several U.S. Patents disclose such presses: U.S. Pat. Nos. 2,902,923; 3,037,445; 3,070,003; 3,086,452; 3,092,017; 3,093,065; 3,111,080; 3,246,597; 3,382,538; 3,518,936; 3,574,891; 3,592,128; 3,721,184; 3,797,891; 4,117,776.
The cage is capable of holding materials therein compressed under high pressures. As the material is fed into the intake chute, pressure is exerted upon it by a series of pressure worms which include screw flights which are mounted on and rotate along with the main drive shaft. The shaft onto which the worm is mounted extends longitudinally through the cage, such that the screw flights, a part of the worm, have a close fit with the press housing and walls of the cage, and, as such, when the shaft is rotated, the flights press the fibrous material toward the discharge end of the press. One or more breaker bars or equivalent stationary members protrude inwardly from the cage in areas not in conflict with the pressure worms to afford a restriction and barrier to helical-type flow of the pressed material. Internally the press is capable of continuously maintaining this high pressure to express the liquid from the fibrous plug.
The purpose of an expressing press is to extract the maximum amount of fluid from the fibrous pressed material. Therefore, the percent of fluids remaining in the exit material is the most pertinent gauge of press efficiency. By increasing the exit pressure, that is, by additional choking of the discharge, or by increasing the feed while maintaining a fixed press speed, the amount of dewatering can be somewhat increased. However, there is a practical limit beyond which overall press efficiency drops due to overstress on certain critical parts, such as the drive train, worms and cage screens. By studying the moisture profile of the exiting fiberous plug, it has been found that the highest percent of fluid is contained in the fiber nearest the rotational axis of the main shaft. Increased liquid removal of entrapped fluids contained within that inner portion of the fibrous plug has been thus far limited to variations in the above operational parameters.
In such presses the outer surfaces of the worm flights forming the screw are often hard-coated to reduce wear on peripheal edges of the flights, where wear is usually most severe. The hard-coating operation is commonly performed by welding a layer of material such as "Stellite", or "Stoody", available commercially, onto the outer peripheral surface of each worm flight, after which the material is ground to bring each worm flight within predetermined final dimensions and surface finish. Frequently, after a screw press has had extensive use, even these hardened edges become worm resulting in a significant reduction in operating efficiency. Such wear is particularly severe when pressing materials contain sand, gravel and other foreign abrasive materials, as they frequently do.
Worm failures may also result from impact of a section of the flight with a relatively large foreign object, which frequently is introduced into the press along with the conveyed feed material. This impact can result in the fracture of all or a part of the flight from the worm body. Any resultant fragment can, in turn, cause further breakage of other flight sections on subsequence worms. This flight fracturing condition is accented by the introduction of stress boundries in the base metal during the weld build-up or hard-coating materials previously described. Improvements described in U.S. Pat. No. 3,596,128 issued to French Mill Oil Machinery comprising replaceable flight surfaces further increase the possibility of this type failure by including into the aforementioned flight sections indentures or anchorments such as bolting holes, slots, cutouts, etc.
When such failures occur, the cage must be opened and the defective worm removed from the pressing chamber. The flights are then replaced or reconditioned by again welding a hard-coating material on their peripheral edges then reground to original dimensions. In the case of flight fracture, the reconditioning process requires that the entire flight be ground down and replaced after which multiple coats of hard-coating are welded to this new base. This reconditioning of the worm flights may require significant downtime of the press if spare replacements are not available, resulting in substantial lost operating time.
The main shaft is essentially supported in cantilever fashion at the drive end, which is also the infeed end of the press. Except for those improvements described in U.S. Pat. No. 3,592,128, whenever a worm or collar requires maintenance or replacement, that member must be removed from the free end of the main shaft, which is located at the discharge end of the press. Therefore all worms and collars located downstream of the member to be serviced must also be slid off the shaft. Although facilitated by removal of the cages, this worm and collar removal and later reassembly procedure is very laborious and time consuming.
Some expressing presses perform the dual function of removing fluids from the fibrous material and serving as the entrance point for conveying the expressed fibrous material into a further process which may operate at considerable temperature or pressure or both. When worm replacement is required under these conditions, worms and collars have to be removed from the shaft through this additional chamber after limited cooling. Workmen entering this chamber to facilitate this removal do so under somewhat difficult and dangerous conditions.
The present invention provides a system for substantially reducing downtime maintainance time and costs. An additional feature includes an improved means for extracting fluids from the fibrous material, thereby increasing operating efficiency of these presses.